WO1987007732A1

WO1987007732A1 - Method for determining the geometry of a multisource seismic wave emission device

Info

Publication number: WO1987007732A1
Application number: PCT/FR1987/000216
Authority: WO
Inventors: Jean Brac
Original assignee: Institut Français Du Petrole
Priority date: 1986-06-13
Filing date: 1987-06-12
Publication date: 1987-12-17
Also published as: US4862422A; FR2600173B1; FR2600173A1; EP0271537B1; CA1284208C; EP0271537A1

Abstract

Method for determining the geometry of a seismic wave emission device comprising a plurality of seismic sources (S11, S12, S21, etc.) towed by a ship (1) with different transverse and longitudinal offsets. It comprises the isolated triggering of each of the sources (S11, S12, S21, etc.), the detection of acoustic waves coming therefrom by means of proximity sensors (C) arranged at known distances from other sources and optionally from the ship or from a seismic flute, and the recording of detecting signals by means of an acquisition system on board the ship (1). After various corrections of the distances and various processings performed on the signals, parameters (propagation time, phase shiftings) representative of intersource distances are determined and, consequently, the relative coordinates of the sources with respect to each other and their positions with respect to other points. The previous measurements are completed by other measurements related to the immersion depth of different sources. Application to ocean seismic prospection.

Description

-AT -

Method for determining the geometry of a multi-source seismic wave emission device

The subject of the invention is a method for determining the geometry of a device for transmitting multi-source seismic waves.

A seismic wave emission device is used to generate seismic waves in a body of water which propagate towards the bottom and are reflected or diffracted by the discontinuities of the submerged subsoil. The waves returned to the surface are picked up by sets of ydrophones distributed along one or more seismic beams towed behind a ship. The seismic signals that they produce are transferred to a seismic laboratory, in order to obtain recordings representative of the configuration of the subsol.

The seismic sources used are, for example, air cannons, water cannons or other sources which are started simultaneously or successively. These combined triggers are aimed at increasing the dissipated acoustic power, at improving the shape of the resultant emission emitted or signed, or even at obtaining a more directional emission diagram. The effect obtained by triggering in sequence very precisely depends on the configuration of the sources used and the time intervals separating their triggers.

It is therefore important to know the geometry of the emission device, that is to say the relative arrangement of all the seismic sources in relation to each other. The emission devices include, for example, several sub-assemblies each consisting of an alignment of several seismic sources arranged at selected intervals along a towing "umbilic" generally comprising towing cables, supply lines for gaseous and / or liquid fluids, power cables, signal transmission lines, etc.

The different sub-assemblies are towed parallel to each other and means of deflection, such as paravanes, are used to keep them separated laterally from each other. Different immersion control systems including, for example, platforms suspended from floats or platforms equipped with adjustable orientation fins and floats, allow sources to be immersed at selected depths. Such deflection and controlled immersion systems are described for example in French patents 2,397,974, 2,420,478 and 2,523,542.

It is easy to measure the Longitudinal distance separating the different sources Along the same umbilic. On the other hand, it is much more difficult to know with precision The real distance of each source compared to the trajectory of the ship during its progression as well as its depth of immersion, at the moment when, the speed of evolution having stabilized and the transceiver device having taken a stable position in the water, seismic prospecting operations can begin. However, the state of the sea, the currents, the towing conditions may change and thus modify the actual arrangement of the transmission-reception assembly from one series of releases to the other. IT FOLLOWS THAT THE SIGNATURE AND / OR THE GUIDELINE DIAGRAM OF THE ENTIRE SET ARE DIFFERENT FOR AN IDENTICAL TIMING OF TRIGGERS.

The recordings obtained from these broadcasts are made more legible in a classic way, by correlations made between the signals picked up by the receiving device and the gLobLe "signature" of the device. To determine this from all points of space, it is necessary to combine in a particular way the individual signatures of each of the sources, taking into account their relative arrangement with respect to each other. A first method, described in French published patent application 2,582,107, consists in establishing a prior catalog containing the signatures of each of the sources of the emission device triggered individually, for a large number of values of the parameters involved in the operation of the sources. , in particular the depth of immersion and, - in operation, taking into account the real values of the parameters measured at the time of the "shots", to combine the corresponding signatures taken in the catalog, possibly corrected, so as to synthesize the synthesis signature infinitely large in all directions of space.

The global signature of the sending device can also be obtained - by measuring the signature of each of the. sources of the transmitting device, in the vicinity of this and to combine them after correcting each of them to take account of operating interactions. The corrections that are made to the signatures depend closely on the relative disposition of the different sources in relation to each other. A process of this type is described, for example, in published European patent application no. 66.423.

The method according to the invention makes it possible to know with precision the geometry of a multi-source seismic emission device whatever its configuration, and therefore allows the implementation of the previous methods of reconstitution of signatures.

The process involves the immersion of several sensors at determined distances from the different sources. It is characterized in that it comprises the isolated triggering of a first seismic source, the detection, by the sensors, of the acoustic waves emanating from the triggered seismic source, the repetition of the two preceding stages successively for each of the other seismic sources of the emission device, the measurement, from the captured signals, of the values taken by at least a parameter representative of the distance traveled by the acoustic waves from each of the sources triggered successively to the various sensors, The combination of the values of the various measured parameters, so as to determine the relative position of the various seismic sources in relation to each other .

The method may include measuring the immersion depth of several seismic sources of the emission device and determining the coordinates of all the seismic sources relative to one of them.

It may also include the immersion of at least one reference sensor, at a predetermined fixed distance from the ship, the determination of the successive values taken by the parameter measured when the various seismic sources are triggered, and the calculation of the position of the different seismic sources with respect to the reference sensor (s).

The method may also include positioning at least one sensor in a fixed position relative to an object towed by the ship, a seismic stream for example, determining the successive values taken by the representative parameter during successive trips of the various seismic sources and the calculation of the position of the different seismic sources in relation to the sensor associated with the object.

The parameter representative of the distance can be the propagation time of the acoustic waves or even the phase shift of the received waves with respect to the emitted waves. Other characteristics and advantages of the process will appear on reading the description of an embodiment given by way of nonlimiting example, with reference to the appended drawings where:

- Figure 1 shows a schematic top view of an arrangement of the different seismic sources of an emission device towed by a ship;

- Figure 2 very schematically shows two sources arranged in a line along the same "ombiLic" of food; and ιrx

- Figure 3 schematically represents the different acoustic paths measured between two alignments from two sources.

An emission device, described by way of example, comprises the following:

15: figure 1 several lines of sources L _* , L- ,, ... L, deployed behind a vessel 1 which tows them. Each alignment comprises several sources S joined by a connecting link or "umbilic" 2 (fig. 2) comprising towing cables, compressed gas or pressurized liquid pipes necessary for the operation of the 20 sources, notably air cannons or water, power cables, signal lines, etc.

The deployment of the sources is carried out by fixing the towing cables associated with each Line of seismic sources (L. to L,) to a Z55 or two cables R .., R- fixed to the ship and offset laterally with respect to the axis LongitudinaL of this one. The lateral spacing of each cable R .., R is obtained by means of a paravane of a known type fixed at its end. One can use for example the profi le divergent described in French patent n ° 2.523.542.

30

The method according to the invention includes the use of sensors (C) placed in the immediate vicinity of each of the seismic sources, at 1 m above for example or even below, as shown. Such a sensor, known as a proximity sensor, is generally used to detect the instant of effective triggering of a seismic source or else to obtain the form of impulse it emits ⁽ signature).

The method comprises the triggering of a first seismic source of emission device and the detection by sensors C associated respectively with the other sources, of at least one parameter representative of the distance traveled by the acoustic waves emanating from the triggered seismic source. . To facilitate the representation, four sources S., S- ,, S ,, S, have been represented in FIG. 3. The source S., being triggered, The sensors C detect The acoustic waves which have propagated to the others sources S, S ,, S ,. The speed of propagation in the water of the waves being known, it is therefore possible, by measuring the delays of the signals received with respect to the signals transmitted, to know the distance d ..- ,, d ... ,, d .. ,, separating La source S. from sources S _? , S, and S,.

The signals detected by the various sensors C are transmitted to an acquisition system arranged on the ship (and not shown) by Transmission lines associated with the "umbiLics" of Link 2 and recorded. We can use an independent acquisition system or possibly the onboard seismic laboratory. The previous operation is repeated successively for the other sources S- ,, S. ,, S ,. Each of them being triggered individually, we detect the arrival of sensors C associated with the other sources, acoustic waves which they emit and we transmit the signals received to the acquisition system on the ship.

When all the seismic sources of the transmitting device have been successively actuated and all the detected signals have been recorded, various corrections are made to take into account the fact that each seismic source does not entirely coincide with the associated proximity sensor C and also to take into account the actual location of the source of the triggered source. In In the case of an air gun dropping a certain volume of pressurized gas into the water, the emission center will be the center of the bubble which imposes by producing acoustic waves.

To facilitate the exact location of the time of arrival of the acoustic waves, a correlation is preferably made between the signal produced by each sensor C in response to the acoustic waves received with the corresponding signal emitted such that it is detected by the sensor C associated with the triggered source.

Once the corrected time intervals have been determined, there are as many ranges of distance values as there are seismic sources. The source S ₂ is associated with the distances d ₂₁ , d- ,, dp, which separate it from the sources S,., S ,,. S ,. Two other tripLets of values d, .., D ,,, and D-., On the one hand and d, .., d, ₂ and d-, on the other hand, are associated respectively with sources S, and S ,.

Then used a method known as triangulation The method of least squares well-known experts, to determine the relative coordinates of all sources pa ^r compared to the one of them.

Determining the position of all sources relative to the ship is possible if one or more sensors C are associated with it (Fig. 1). On ^'has so each trigger seismic source, at least a measure of the distance separating it from the ship. With two sensors C in a fixed position relative to the ship, it is possible to determine the coordinates of all the seismic sources of the emission device.

We can also determine the position of the emission device in relation to other objects and in particular at the head of a seismic beam also towed by the ship. Other sensors contained in the head sections of the flute are then used to ^• receive impulses emanating successively from different sources.

If the seismic sources of the emission device are not submerged to an identical depth, their respective immersion depths 5 are measured and they are transmitted to the acquisition system.

The use of telemetry data and depth values makes it possible to determine the coordinates in space of all sources.

ÎT The measurement of the immersion depths is advantageously carried out using the proximity sensors of the various seismic sources, in the manner indicated in the aforementioned published French patent application 2,582,107. Each proximity sensor is associated with an application chain, the gain of which is likely to take two

15 very different gain values, a first for the measurement of the immersipn depth, a second weaker for the detection of acoustic waves coming from successively actuated sources.

The method according to the invention can be implemented in conjunction with that described in the aforementioned patent application and relating to the determination of the "signature" collected over a long distance from a multi-source emission device. This global signature is obtained by establishing a catalog of the particular signatures of all the sources triggered one after the other, these signatures being detected by proximity sensors such as the C sensors.

It would not go beyond the scope of the invention to replace the measurement of the times, of propagation of the acoustic waves by that of the 3 dép phase shift of the signals received by the various proximity sensors with respect to the signals successively emitted by each of the sources. This variant can be applied in particular to vibration sources. We would not go beyond the scope of the invention by using a number of sensors different from that of seismic sources. The same sensor can be associated, for example, with several sources, provided that the distances which separate it from these are well determined. In the same way as above, the calculation of the inter-source distances is carried out after introduction of the corrections to take account of the actual distance of each sensor from the associated seismic sources.

Claims

1. - Method for determining the geometry of a seismic emission device made up of a plurality of seismic sources immersed in water at a distance from each other and towed by a ship comprising the immersion of several sensors (C ⁾ at determined distances from the various seismic sources, characterized in C that it comprises the following stages:

- The isolated triggering of a first seismic source ⁽ S _* ....

- Detection by sensors (C) of the acoustic waves emanating from the triggered seismic source;

- The repetition of the two preceding stages successively for

- each of the other seismic sources of the emission device;

- The measurement, from the signals received, of the values taken by at least one parameter and representative of the distance traveled by the acoustic waves from each of the sources triggered successively to the various sensors; and

- The combination of the values of different parameters measured so as to determine the relative position of the different seismic sources with respect to each other.

2. - Method according to claim 1, characterized in that it comprises the measurement of the immersion depth of several seismic sources of the emission device and the determination of the coordinates of all the seismic sources relative to one of between them.

3. - Method according to claim 1 or 2, characterized in that On has at least one reference sensor in a fixed position relative to the ship, the successive values taken by said parameter are determined during the successive trips of the different seismic sources, and the position of the different seismic sources is calculated relative to the reference sensor.

4. - Method according to claim 1, characterized in that the parameter representative of the distance is the propagation time of the acoustic waves.

5. - Method according to claim 1, characterized in that the parameter representative of the distance is the phase shift between the waves picked up and the waves emitted by the triggered seismic source.

6. - Method according to claim 2, characterized in that each sensor is adapted to detect the acoustic waves received and the depth of immersion of the associated seismic source.

7. „- Method according to claim 3, characterized in that each reference sensor is fixed to the hull of the ship.

8. - Method according to one of claims 1 to 3, characterized in that there is at least one sensor in a fixed position relative to an object being towed by the ship, the successive values taken by said parameter are determined during the successive trips of the different sources and the position of the different seismic sources is calculated relative to the sensor associated with said object.

9. - Method according to claim 8, characterized in that said object is a seismic stream.